In consideration of various factors such as environmental protection, safety in the working environment, resource saving, cost etc. there has been a large trend in recent years away from supplying adhesives of solution type (using an organic solvent as solvent medium) and more to supplying adhesives in non-solvent type (which do not use organic solvents). Amongst these non-solvent type, the use of water-dispersion type, i.e. so-called emulsion type pressure-sensitive adhesives has been favoured in consideration of their general purposeness and workability.
However, with these water-dispersion type pressure-sensitive adhesives, it is necessary to have a dispersant such as an emulsifying agent in order to disperse the pressure-sensitive adhesive polymer into the aqueous medium, and since such dispersants are hydrophilic materials, there has been the major problem that the water resistance of such adhesives is poor compared to solvent-type adhesives. In addition to the reduction in water resistance, the presence of the dispersant also has a bad affect on the adhesive strength itself, balance between cohesion and adhesion, adhesiveness to a substrate, transparency etc. and at present there has not been developed a water-dispersion type pressure-sensitive adhesive which displays levels of water resistance and adhesive properties on a par with solvent types.
In an attempt to resolve the above-mentioned problems with water-dispersion type pressure-sensitive adhesives, there have been reported methods of improving the dispersion itself or adopting cross-linking means etc. As one such cross-linking means, there is known a method involving forming the dispersion by the copolymerization of monomers having functional groups when producing the pressure-sensitive adhesive polymer, and then adding a cross-linking agent which reacts with these functional groups. However, if the cross-linking is carried out to such an extent that a sufficient degree of water resistance can be realized, there then arises the problem that the flowability, which is necessary for wetting the adherend, is reduced with a consequent lowering in adhesive strength.
On the other hand, in Japanese Patent Application Publication No. H-1-221402, the applicant of this invention discloses that a water-dispersion type pressure-sensitive adhesive employing as an emulsifying agent a polymer having a specific structure is excellent in terms of several physical properties such as adhesive strength, water resistance, moisture resistance etc., and that by using a monomer having functional groups which can react with the carboxyl groups of the emulsifying agent as the monomer for the pressure-sensitive adhesive polymer, the emulsifying agent and the pressure-sensitive adhesive polymer react with each other after the formation of a layer of the pressure-sensitive adhesive i.e. after the adhesive is applied and dried to create a film, with a resulting further increase in the water resistance. However, when compared with the superior water resistance displayed by solvent-type pressure-sensitive adhesives, the pressure-sensitive adhesives disclosed in Japanese Patent Application Publication No. H-1-221402 cannot be described as satisfactory. The inventors of the present invention made continued studies with a view to increasing the water resistance, and found that it is not desirable to just simply increase the amount of functional groups in the pressure-sensitive adhesive polymer, since the result is that the adhesive strength is reduced thereby causing problems with respect to the balance between water resistance and adhesive strength, and that the polymerization stability is lowered.
In Japanese Patent Publication No. H-2-3432, there is disclosed an invention which involves synthesizing a water dispersion of the pressure-sensitive adhesive polymer by an emulsion polymerization method using an emulsifying agent having functional groups which can react with isocyanate groups, adding to this water dispersion a non-water-miscible organic solution of a polyisocyanate compound and then reacting the two in the water-dispersed state until no remaining isocyanate groups can be detected The pressure-sensitive adhesive of that invention is disclosed to be excellent in terms of adhesive strength and retention of moisture resistance. However, since the method involves reacting the functional groups of the emulsifying agent and the isocyanate compound in the water-dispersed state, then even when both have been reacted together until no isocyanate groups are detected, it is still necessary that many of the hydrophilic groups in the emulsifying agent still remain in order that a stable water-dispersed state be maintained. In other words, if all the hydrophilic groups in the emulsifying agent react with the isocyanate groups, then it becomes impossible to maintain a stable water-dispersed state. However, in the invention of Japanese Patent Publication No. H-2-3432, there was no recognition of the problem associated with reacting all the hydrophilic functional groups in the emulsifying agent.
Furthermore, although it is disclosed that the pressure-sensitive adhesive of that invention is excellent in terms of adhesive strength and retention of moisture resistance, the value for retention of water resistance disclosed therein is a value which was measured at a level of humidity of 65% at the most. When compared with severe water resistance tests involving immersion in water, this is a value of rather gentle condition, and the adhesive can hardly be described as being excellent in terms of "water resistance". Furthermore, since isocyanate groups are highly reactive with water, they will be consumed by the water constituting the dispersing medium, causing problems with respect to reproducibility of performance such as adhesion strength, retention of moisture resistance.
It is therefore an objective of the present invention to significantly increase the water resistance of water-dispersion type pressure-sensitive adhesives and develop a water-dispersion type pressure-sensitive adhesive composition having a level of water resistance equal to or better than that of solvent-type pressure-sensitive adhesive compositions, and a method for producing the same. It is a further objective of the present invention to provide a pressure-sensitive adhesive product which is excellent in terms of water resistance, moisture resistance and balance between cohesive strength and adhesive strength and which displays excellent adhesiveness to a substrate.